1. Field of the Invention
This invention relates to a magnetic recording and reproducing method, applicable to a magnetic recording and reproducing apparatus such as a video tape recorder (VTR, hereafter).
2. Description of the Prior Art
In the most popularly used conventional magnetic recording and reproducing method, used for such as rotary head type VTRs, video signals are recorded on video tracks by rotary video heads having different azimuth angles and audio signals are recorded on audio tracks by static audio heads. Almost all area of the tape surface is used for the video and audio tracks. Accordingly, efforts for improving the recording density at the surface have been made by reducing the wavelength of the signal to be recorded or the track width.
Recently, on the other hand, a method for recording a frequency-modulated audio signal on a video track has been developed to improve the performance of audio signal recording. Frequency allocation in this recording method is shown in FIG. 1, and a sectional view of the recorded magnetic tape is shown in FIG. 2. Referring to FIG. 1, reference numeral 1 is a frequency-modulated luminance signal including the synchronous signal; 2 is a chrominance signal with its frequency converted to low frequency band; and 3 is a frequency-modulated audio signal. The audio signal 3 is allocated between the frequency-converted chrominance signal 2 and the frequency-modulated luminance signal 1, and recorded on a magnetic tape by an audio head having a different azimuth from that of a video head.
Referring to FIG. 2, the frequency-modulated audio signal is first recorded in a deep layer 4a of a magnetic layer 4 by a recording current larger than a recording current for recording the video signal, and thereafter the video signal is recorded in a shallow layer 4b. Reference numeral 5 is a basefilm. Since the frequency-modulated audio signal is recorded, signal-to-noise ratio of the audio signal is high. Further, the quality of the audio signal is not degraded even if the tape speed is lowered to, for example, a third of the normal tape speed. This feature is advantageous for long-time recording. Also, it is significant that the deep layer of the magnetic layer is used for recording.
However, there are some problems in the above described conventional method.
First, since a large current must be applied to the audio recording head to record the frequency-modulated audio signal in a deep layer, the recordable frequency band is restricted to relatively lower frequency band. Consequently, the method cannot be used for the pulse-code modulation (PCM) recording system which provides a quality better than the frequency-modulation (FM) recording system, because the PCM recording requires a frequency band wider than that of the FM recording. Further, during reproducing the recorded video signal, a part of the audio signal recorded in the deep layer is also reproduced to interfere with the reproduced video signal. Further, the large current for recording the audio signal needs a large scale current supply circuit, and badly influences other video circuits.
Second, the so-called "after-recording" is impossible, which records or erases only audio signals on the magnetic tape on which video signals have already been recorded. Since the audio signal is recorded in the deep layer of the magnetic tape, it is impossible to erase or record only the audio signal the audio signal without erasing or affecting the video signal recorded in the shallow layer.
As described above, the multiplex recording method using a deep layer of magnetic layers of a magnetic tape has such problems that it is difficult to increase the record density and that it is impossible to rewrite the informations in the deep layer.